Hello Future Biologist! Welcome to the World of Control Systems
Welcome to the "Control" section of your Biology studies! This chapter is incredibly exciting because it explains how every living thing—from a plant shoot turning towards the sun to your heart beating—manages to react perfectly to its ever-changing environment.
Think of your body as a super-advanced smart home: sensors detect changes (the stimulus), wires send signals (nerves/hormones), and devices turn on or off (the response). Mastering this topic is key to understanding complex physiological processes like reflexes, vision, and maintaining a stable internal temperature (homeostasis).
I. Stimulus, Response, and Simple Control
A. The Simple Reflex Arc
A reflex is a rapid, involuntary response that bypasses the conscious part of the brain. It is vital for preventing harm.
A simple reflex arc typically involves three neurones (nerve cells):
1. Sensory Neurone: Carries the impulse from the receptor (e.g., skin) to the Central Nervous System (CNS, usually the spinal cord).
2. Relay Neurone (or Interneurone): Found entirely within the CNS. It links the sensory neurone to the motor neurone.
3. Motor Neurone: Carries the impulse away from the CNS to the effector (e.g., muscle or gland) to produce the response.
The importance of simple reflexes is rapid, immediate action to avoid damage (e.g., pulling your hand away from a hot stove).
B. Simple Behavioural Responses: Taxes and Kineses
These are basic, non-learned movements that help maintain an organism in a favourable environment, increasing its chance of survival.
• Taxes (Taxis): A directional movement in response to a stimulus.
• Example: A woodlouse moving towards darkness (positive phototaxis) to find a moist environment.
• Kineses (Kinesis): A non-directional, random change in the speed or rate of turning in response to a stimulus intensity.
• Example: A woodlouse moves faster and turns more often when it hits a dry, unfavourable patch until it randomly enters a damp spot, where it slows down and stays.
Key Takeaway: Reflexes are fast, wired reactions inside the body; Taxes and Kineses are simple whole-body movements driven by survival instinct.
II. Detection: Receptors
Receptors are specialized cells or organs that detect stimuli. A key principle is that receptors respond only to specific stimuli. When stimulated, they establish an electrical change called the generator potential.
A. The Pacinian Corpuscle (Pressure Receptor)
The Pacinian corpuscle is an example of a receptor found deep in the skin, responsible for detecting pressure and vibration.
Basic Structure:
• It contains the end of a sensory neurone surrounded by layers of connective tissue (lamellae) separated by fluid.
Mechanism of Action:
- In its resting state, the membrane of the sensory neurone end is impermeable to sodium ions (\(Na^{+}\)), maintaining a resting potential.
- When pressure is applied (the specific stimulus), the corpuscle is deformed (squashed).
- This deformation stretches the plasma membrane of the sensory neurone ending.
- Stretching causes the stretch-mediated sodium ion channels to open.
- \(Na^{+}\) ions rapidly rush into the neurone, causing a depolarisation (making the inside more positive).
- This small depolarisation is the generator potential. If this potential is large enough (i.e., the pressure is strong enough) to reach the threshold potential, an action potential (nerve impulse) is generated and transmitted along the sensory neurone.
B. The Human Retina (Light Receptor)
The retina contains two types of photoreceptor cells: rods and cones. Differences in their structure and connections explain differences in vision.
| Feature | Rod Cells | Cone Cells |
|---|---|---|
| Optical Pigment | Rhodopsin (highly sensitive to light) | Iodopsin (three types, sensitive to red, green, or blue light) |
| Sensitivity to Light | Very high (Work well in dim light) | Lower (Require bright light) |
| Colour Vision | No colour detection (Only shades of grey) | Provide colour vision |
| Visual Acuity (Detail) | Low (Many rods share one bipolar cell, resulting in summation and poor detail) | High (Usually one cone cell per bipolar cell, allowing visual acuity) |
Did you know? The reason you can't see colours clearly at night is because only your highly sensitive Rod cells are functioning, while the Cone cells need more light energy to be activated.
III. Transmission: Nerve Impulses
A. The Structure of a Myelinated Motor Neurone
Motor neurones transmit impulses from the CNS to effectors (muscles or glands).
• Myelin Sheath: A fatty layer (made of Schwann cells) insulating the axon. • Nodes of Ranvier: Gaps in the myelin sheath. • Myelination is a key factor in speeding up impulse conduction.
B. Establishing the Resting Potential
When a neurone is not transmitting an impulse, it maintains a resting potential (around -70 mV).
This is achieved by:
• Sodium-Potassium Pumps: Actively transport \(3 Na^{+}\) ions out of the axon for every \(2 K^{+}\) ions in. This requires ATP.
• Differential Membrane Permeability: The membrane is far more permeable to \(K^{+}\) ions (which leak out down their concentration gradient) than to \(Na^{+}\) ions.
• This creates an electrochemical gradient: the outside is positive relative to the inside.
C. Generating an Action Potential
An action potential is a rapid, temporary change in the membrane potential from negative (resting) to positive (depolarisation) and back to negative (repolarisation).
Step-by-Step Action Potential:
1. Depolarisation: If the generator potential reaches the threshold (typically -55 mV), voltage-gated \(Na^{+}\) channels open rapidly, causing a massive influx of positive \(Na^{+}\) ions. The internal potential becomes positive (up to +40 mV).
2. Repolarisation: The \(Na^{+}\) channels close, and voltage-gated \(K^{+}\) channels open. \(K^{+}\) ions rush out of the axon, restoring the negative internal charge.
3. Hyperpolarisation: \(K^{+}\) channels are slow to close, causing a brief period where the potential drops below the resting potential (more negative than -70 mV).
4. Resting Potential Re-established: The sodium-potassium pump actively restores the ion concentrations back to the original resting state.
The All-or-Nothing Principle
Action potentials are all-or-nothing events. If the threshold is reached, an action potential of a constant size is generated. If the threshold is not reached, no impulse is fired.
Analogy: Turning on a light switch. It's either ON (full impulse) or OFF (no impulse). You can't have a half-on light switch.
The Refractory Period
This is a short period immediately after an action potential when the axon cannot generate another impulse.
Importance:
• Ensures impulses are discrete (separate) and unidirectional (travel in one direction only).
• Limits the frequency of impulse transmission (how many impulses can pass per second).
D. Factors Affecting Speed of Conduction
The speed at which an impulse travels depends on three main factors:
• Myelination (Saltatory Conduction): In myelinated neurones, the impulse "jumps" from one Node of Ranvier to the next, significantly speeding up conduction. This is called saltatory conduction.
• Axon Diameter: A thicker axon has less resistance to the flow of ions, meaning the impulse travels faster.
• Temperature: Higher temperature increases the rate of diffusion and respiration (affecting the \(Na^{+}\)-K+ pump), thus increasing conduction speed (up to a point, before denaturation occurs).
Quick Review: Myelination is the biggest factor, enabling saltatory conduction.
IV. Communication: Synaptic Transmission
A. Structure of a Cholinergic Synapse
A synapse is the junction between two neurones, or between a neurone and an effector (like a muscle, called a neuromuscular junction). Cholinergic synapses use the neurotransmitter acetylcholine (ACh).
Key Features to Explain Synaptic Function:
• Unidirectionality: Impulses only travel in one direction—from the presynaptic membrane to the postsynaptic membrane. This is because neurotransmitter vesicles are only found in the presynaptic knob, and receptors are only on the postsynaptic membrane.
• Summation: Synapses allow multiple impulses to be integrated.
- Temporal Summation: A high frequency of impulses from a single presynaptic neurone builds up the concentration of neurotransmitter until the threshold is reached in the postsynaptic cell.
- Spatial Summation: Impulses arrive simultaneously from several different presynaptic neurones, all contributing neurotransmitter to trigger an action potential in the postsynaptic cell.
• Inhibition: Inhibitory synapses make the postsynaptic membrane less likely to fire an impulse. They achieve this by causing hyperpolarisation (making the inside even more negative), requiring an even larger stimulus to reach the threshold.
Predicting Effects of Drugs and Toxins:
Drugs and toxins often interfere with synaptic transmission. For example:
• A toxin that blocks ACh receptors will prevent the impulse from crossing the synapse, leading to muscle paralysis.
• A drug that prevents the breakdown of ACh will cause constant stimulation of the postsynaptic membrane.
Key Takeaway: Synapses are not just switches; they are decision-making junctions that can amplify, inhibit, or integrate signals.
V. Response: Skeletal Muscles as Effectors
A. Gross and Microscopic Structure
Skeletal muscle cells (fibres) are made up of bundles of filaments called myofibrils. The myofibril displays a striped pattern due to the arrangement of two types of protein filament:
• Actin: Thinner filaments.
• Myosin: Thicker filaments.
These filaments slide past each other during contraction, known as the sliding filament theory.
B. The Sliding Filament Theory: Roles of Key Molecules
Muscle contraction is an energy-intensive cycle involving five crucial components:
1. Calcium Ions (\(Ca^{2+}\)): Released from the sarcoplasmic reticulum upon arrival of an action potential. They bind to troponin.
2. Tropomyosin: A protein wrapped around the actin filament. In the resting state, it blocks the myosin binding sites. When Ca\(^{2+}\) binds to troponin, tropomyosin moves, exposing the sites.
3. Myosin: The heads of the myosin filament bind to the exposed sites on actin, forming an actomyosin bridge.
4. ATP: Provides the energy required for two steps: i) cocking the myosin head (allowing it to pull the actin filament) and ii) breaking the actomyosin bridge so the cycle can repeat.
5. Actin: Pulled towards the centre of the sarcomere by the repeated action of the myosin heads, causing the muscle to shorten (contract).
C. Muscle Types and Energy Supply
• Antagonistic Pairs: Muscles work in opposing pairs (e.g., biceps and triceps) acting against an incompressible skeleton (a skeleton that doesn't easily change shape). One contracts while the other relaxes.
• Energy Supply: Muscles need rapid, immediate ATP.
- The immediate source is ATP stored in the muscle.
-
If ATP runs low, Phosphocreatine regenerates ATP quickly and anaerobically:
Phosphocreatine + ADP → Creatine + ATP
Slow vs. Fast Skeletal Muscle Fibres
• Slow Twitch Fibres: Adapted for sustained, prolonged activity (endurance). They rely on aerobic respiration, have many mitochondria, and a good blood supply (appear red due to high myoglobin).
• Fast Twitch Fibres: Adapted for short bursts of powerful activity (speed). They rely on anaerobic respiration, have few mitochondria, and contain stores of phosphocreatine.
VI. Control Systems in Plants (Growth Factors)
Plants rely on chemical signals called plant growth substances (or hormones) to control their activities, especially growth and response to the environment. These substances are effective even in very low concentrations.
A. Tropisms and Auxins
Tropisms are growth responses to a directional stimulus that orient the plant in a favourable environment (e.g., roots grow down, shoots grow up and towards light).
• Auxins (e.g., Indoleacetic acid, IAA) are key growth hormones.
• They promote cell elongation in the shoot, but inhibit cell elongation in the root.
This dual effect is crucial:
• In the shoot, IAA moves to the shaded side, promoting faster growth there, causing the shoot to bend toward the light (phototropism).
• In the root, IAA moves to the underside (due to gravity). Because roots are extremely sensitive, this concentration inhibits elongation on the underside, causing the upper side to grow faster, making the root bend down (geotropism).
B. Ethene and Abscisic Acid (ABA)
• Ethene: A gas involved in fruit ripening. This is exploited commercially: climacteric fruits (like bananas) are picked green and unripe, shipped, and then artificially ripened with ethene just before sale.
• Abscisic Acid (ABA): The "stress" hormone, crucial for responding to drought.
ABA and Stomatal Closure
When a plant is water-stressed (or the roots sense water deficiency), ABA is released.
1. ABA attaches to receptors on the guard cell membrane.
2. This stimulates the rapid transport (efflux) of potassium (\(K^{+}\)) and chloride (\(Cl^{-}\)) ions out of the guard cells.
3. The loss of these solutes raises the water potential inside the guard cells.
4. Water moves out of the guard cells by osmosis, causing them to shrink and the stomata to close, reducing water loss via transpiration.
Key Takeaway: Plant growth factors regulate everything from directional growth (tropisms) to critical survival mechanisms (stomatal control).
VII. Maintaining Stability: Homeostasis and Feedback
A. The Principles of Homeostasis
Homeostasis is the maintenance of a stable internal environment within restricted limits, regardless of external changes.
Why is it important?
• Enzyme Activity: Enzymes are proteins. Maintaining a stable core temperature and blood pH is vital because deviations cause denaturation, stopping metabolic reactions.
• Example: If blood pH drops too low, respiratory enzymes stop working efficiently.
B. Negative and Positive Feedback
Homeostasis relies heavily on feedback mechanisms.
• Negative Feedback: The primary mechanism of homeostasis. It works to restore the system to its original, ideal level (set point).
• Analogy: A thermostat. If the temperature rises, the heater is turned off to bring it back down.
• The body often employs separate mechanisms controlling departures in different directions (e.g., one mechanism to raise temperature, one to lower it), giving a greater degree of control.
• Positive Feedback: A mechanism that results in a greater departure from the original level, often speeding up a process.
• Example: Contractions during childbirth. The stimulus (contractions) causes the release of oxytocin, which increases the contractions.
• Warning: Positive feedback is often associated with the breakdown of control systems (e.g., rapid fever leading to uncontrollable temperature rise).
Quick Review: Negative feedback = Stabilise (Good); Positive feedback = Amplify (Sometimes good, often bad).
VIII. Case Study: Hormonal Control of Blood Glucose
A. Importance and Liver Functions
Maintaining stable blood glucose concentration is crucial because glucose is the immediate source of energy (required for respiration) and affects the water potential of the blood (high glucose lowers blood water potential, leading to excessive water loss via urine).
The liver plays a central role, performing three key inter-conversions:
• Glycogenesis: Glucose converted to glycogen (storage).
• Glycogenolysis: Glycogen converted back to glucose (release).
• Gluconeogenesis: Glucose synthesis from non-carbohydrate sources (e.g., glycerol or amino acids).
B. Hormones Involved in Glucose Control
Three main protein hormones manage blood glucose, released by the pancreas (insulin, glucagon) or adrenal glands (adrenaline).
Insulin (Lowers Blood Glucose)
Released by Beta cells in the pancreas when glucose is too high. Acts by:
• Attaching to receptors on target cells (especially liver and muscle).
• Controlling glucose uptake by regulating the inclusion of channel proteins in the surface membranes of target cells.
• Activating enzymes involved in glycogenesis (glucose to glycogen).
Glucagon (Raises Blood Glucose)
Released by Alpha cells in the pancreas when glucose is too low. Acts by:
• Attaching to receptors on target cells (primarily liver).
• Activating enzymes involved in glycogenolysis (glycogen to glucose).
• Activating enzymes involved in gluconeogenesis (making glucose from non-carbohydrates).
Adrenaline (Raises Blood Glucose)
Released by the adrenal glands during stress or excitement (Fight or Flight). It acts similarly to glucagon by attaching to liver cell receptors and activating enzymes involved in glycogenolysis.
C. The Second Messenger Model (Glucagon and Adrenaline)
Glucagon and Adrenaline are protein hormones and cannot pass through the plasma membrane. They use a system called the Second Messenger Model:
- The hormone (the first messenger) attaches to a specific receptor on the target cell surface.
- The binding activates an enzyme inside the cell called adenyl cyclase.
- Adenyl cyclase converts ATP into cyclic AMP (cAMP)—this is the second messenger.
- cAMP activates other enzymes, specifically protein kinase.
- Protein kinase then triggers a cascade of reactions (e.g., activating enzymes for glycogenolysis) leading to the final response (glucose release).
D. Diabetes Mellitus
A condition characterized by the inability to control blood glucose concentration within restricted limits.
• Type 1 Diabetes (Insulin Dependent): Usually caused by an autoimmune response destroying the Beta cells in the pancreas. The body cannot produce insulin.
• Control: Managed primarily by injecting insulin and dietary manipulation.
• Type 2 Diabetes (Insulin Resistant): Often linked to lifestyle and obesity. Target cells lose their sensitivity to insulin (down-regulation of receptors).
• Control: Managed primarily through diet changes, exercise, and sometimes drugs or insulin.
Understanding the symptoms: High blood glucose (hyperglycaemia) leads to poor blood water potential. This causes symptoms like excessive thirst and fatigue (lack of glucose in cells for energy).
IX. Control of Heart Rate
The heart muscle is myogenic, meaning it generates its own electrical signal.
1. The electrical impulse begins at the Sinoatrial Node (SAN), often called the heart's pacemaker, causing atrial contraction.
2. The impulse travels to the Atrioventricular Node (AVN), which introduces a slight delay.
3. The impulse then travels down the septum via the Bundle of His and spreads rapidly through the ventricular walls using Purkinje tissue, causing ventricular contraction from the apex upwards.
External Regulation: The rate of this myogenic rhythm is controlled by the autonomic nervous system (ANS), which responds to information from receptors:
• Chemoreceptors: Located in the aorta and carotid arteries. Detect changes in blood chemistry (specifically pH due to \(CO_{2}\) concentration).
• High \(CO_{2}\) = Lower pH = Need for faster heart rate to remove \(CO_{2}\).
• Pressure Receptors: Located in the aorta and vena cava. Detect changes in blood pressure.
• Low blood pressure = Need for faster heart rate to increase cardiac output.
The ANS then signals the SAN via sympathetic (accelerates rate) or parasympathetic (decelerates rate) nerves to adjust the heart rate accordingly.
Final Key Takeaway: Control systems ensure survival and optimal function by linking stimulus detection, signal transmission, and effector response through incredibly precise biological wiring and chemistry.